Track Pi session id alongside pid in Pinet agent roster and diagnostics

[gugu91]

Maintainer-requested follow-up from Slack thread 1777815587.943179.

Context

During ghost/recovery work, the broker could see agent pid and Pinet agent id in the roster, but needed to inspect the broker DB manually to find the original Pi session/stable id for resurrecting a specific worker. Tracking only pid is not enough for safe resurrection/reconnect workflows.

Useful fields observed manually:

• Agent id: 7daefa35-d794-4196-887d-fc0964692389
• PID before ghost: 27541; resurrected PID: 42791
• Stable/session id: Wills-Mac-mini.local:session:/Users/will/.pi/agent/sessions/...jsonl

Ask

Track and expose the Pi session id / stable session file identity alongside pid in Pinet agent roster/status/diagnostics so brokers can safely identify and resume the exact Pi session for a worker.

Acceptance criteria

• Pinet agent listing/status surfaces include a concise session/stable-id indicator alongside pid, without leaking sensitive paths unnecessarily in Slack-facing output.
• Broker/RALPH diagnostics can identify the exact session/stable id needed for resurrection.
• Tests cover the roster/formatting path and any metadata persistence changes.
• Document safe handling/redaction expectations for session paths if shown in Slack/Pinet output.

No secret values should be logged or exposed.